1. Field of the Invention
The present invention relates to a thin film transistor using an oxide semiconductor for a channel layer, and a display device and an electronic device each using the thin film transistor.
2. Description of the Related Art
In recent years, with the aim of application of a thin film transistor (TFT), a light emitting device, a transparent conductive film, and the like to electronic devices, research and development on oxide semiconductors such as zinc oxide and indium gallium zinc oxide is actively performed. It is known that, in the case of using such oxide semiconductor to an active layer (channel) of a TFT, as compared with the case of using amorphous silicon which is generally used for a liquid crystal display or the like, the electron mobility is higher, and more excellent electric characteristics are displayed. There are also advantages such that the high mobility is expected even at a low temperature around the room temperature, so that the oxide semiconductor is positively developed. A TFT using such an oxide semiconductor layer of a bottom-gate type and that of a top-gate type are reported (see, for example, WO2005-088726).
A bottom-gate-type TFT having a structure that, for example, a gate electrode is provided on a substrate, and a thin film layer of oxide semiconductor is formed over the gate electrode with a gate insulating film in between is known (see, for example, Japanese Unexamined Patent Application Publication No. 2007-194594). The structure is similar to a bottom-gate-type TFT structure using amorphous silicon for the channel which is presently commercially used. Consequently, at the time of manufacturing a TFT of oxide semiconductor, an existing TFT manufacturing process using amorphous silicon may be easily used. A TFT using the oxide semiconductor for the channel is being commercially available.